(1) Field of the Invention
This invention relates to solar energy absorber devices. More particularly, the invention relates to solar energy absorber devices of a thermal rather than photovoltaic nature that employ solar energy collectors directly heated by sunlight and that, in turn, heat a fluid used to extract heat energy from the device.
(2) Description of the Related Art
Solar energy absorber devices of this kind are generally produced in the form of relatively thin panels that may be mounted on the roofs of buildings or in other convenient locations selected to receive direct sunlight during a substantial part of the day. The panels generally consist of a thin flat box having a transparent wall normally made of glass or a transparent plastics material with the remainder made of a metal, wood or plastics that is preferably insulated against heat loss. The box contains a solar energy collector in the form of a flat plate set back a few centimeters from the transparent wall and having a front surface colored black or coated with a special coating that absorbs incident sunlight and converts it to heat. The heat thereby generated is extracted from the rear surface of the collector plate via a metal tube (usually made of copper) arranged in a serpentine fashion through which a heat extraction fluid, normally an aqueous liquid, e.g. water or a mixture of water and propylene glycol (antifreeze), is caused to flow. Under normal conditions, the heat extraction fluid may reach temperatures as high as 100° C. due to the transfer of thermal energy from the collector plate to the metal tubes. The amount of energy captured depends on the size of the collector plate, the strength of the incident sunlight and the overall design of the device that may, if not planned carefully, lead to unwanted heat losses. Collector sizes are normally limited by handling and aesthetic considerations, particularly when the devices are intended to be roof-mounted and therefore highly visible. To minimize heat losses, such devices are generally thermally insulated, as mentioned above, and the box is normally closed and sealed to prevent convective heat loss and to protect the outer surface of the absorber from adverse effects of the weather. In use, therefore, still air provides a blanketing effect so that heat builds up within the box.
The collector plate itself must have good heat conductivity so that heat collected at the front surface readily passes through the panel to the rear surface where it is extracted. For this reason, it is normal for the panel to be made of a heat-conductive metal, e.g. aluminum, copper or alloys of these metals. The serpentine metal tube is generally directly attached to the rear surface of the collector plate and this may be done by various methods, e.g. physical forming, laser or ultrasonic welding, or soldering. In the case of laser welding (which produces good joints) a laser-weldable protective layer may be provided on the rear side of the plate. This offers protection for the plate and makes the welding process easier. However, discontinuous welds (i.e. so-called stitch welds) are often used for connecting the metal tube to the collector plate in order to reduce costs. The tube is therefore attached to the plate only at spaced intervals and the weld bead does not run along the entire length of the metal tube. This creates efficiency losses during the thermal energy transfer and, indeed, the use of welding for attachment of the tube is not highly energy efficient even when the welded bead does extend fully along the metal tube. Not only is the area of contact rather limited compared to the total surface area of the collector plate, but the material forming the weld may not have good thermal conductivity, and the heat has to pass through the wall of the metal tube before it heats the heat extraction fluid contained within.
Solar devices of this kind should be designed to be as maintenance-free as possible, and to have a long working life. The devices do not contain any moving parts that require routine maintenance, but corrosion may occur due to adverse effects of the heat extraction fluid. Unfortunately, materials that have good strength and properties that make them easy to fabricate into solar collectors or heat removal tubes often do not have good resistance to corrosion. To overcome this, attempts have been made to coat inner surfaces of the tubes or channels used for conveying the heat extraction fluids with corrosion-resistant materials, e.g. metal oxides, or to clad metal surfaces with layers of sacrificial alloys. Such approaches have been described, for example, in U.S. Pat. No. 4,178,990 which issued to James M. Popplewell on Dec. 18, 1979. However, coating with metal oxides was found to be unsatisfactory in completely precluding pitting corrosion, and the use of clad metals (e.g. aluminum clad with aluminum-zinc alloy) was found to be complex and costly due to the fact that two clad sheets must be bonded together. It was also found that protective claddings provided only limited protection since they were rapidly consumed by corrosion leaving an unprotected core surface, and corrosion occurred at the bonded portions, thereby resulting in penetration along the bonded interface and leakage of the heat transfer fluid. Popplewell preferred the provision of a getter substance having a high affinity for corrosive metal ions so that such ions would be removed from heat exchange fluid before such fluid passed through the solar collector apparatus.
In U.S. Pat. No. 4,062,350, which issued to Gerald C. Reed on Dec. 13, 1977, a solar absorber is described as having a base sheet made of stainless steel and an absorber sheet made of stainless steel having at least one surface coated with copper. The use of stainless steel can provide solar collector devices that are quite heavy. The patentee discourages the use of aluminum in such devices because aluminum is subject to corrosion when exposed to typical city water and is even more susceptible to corrosion when exposed to swimming pool water. Copper is employed because of its good thermal conductivity and corrosion resistance, but the patentee points out that the cost of copper is relatively high and it is a metal that might become scarce in the future.
U.S. Pat. No. 4,292,955 issued to Harold W. Smith on Oct. 6, 1981 discloses a solar energy collector in which the solar energy collector is said to be made of a metal having high corrosion resistance and heat transfer properties, for example copper.
European Patent Application EP 07100563.1, published as European patent publication no. EP 1811245 on Jul. 25, 2007, describes the construction of a solar collector from two sheets of aluminum. The first aluminum sheet is an absorber plate coated with a radiation absorbing coating. The second aluminum sheet, the lower sheet, comprises stampings which help form the channels through which the cooling fluid flows. The absorber plate and pre-stamped lower sheet are joined by soldering to form the closed channels. It is suggested that alloys of the Aluminum Association 3XXX series may be used, among others.
There is a need for an improved design for devices of this kind so that good efficiency of heat extraction may be obtained while minimizing corrosion of the materials of construction.